The present invention relates in general to the structure of color cathode ray tubes, more particularly, the invention relates to the structure of the tube's inner magnetic shield.
FIG. 1 is a partially cut-away side view of a color cathode ray tube to which the present invention pertains. A funnel 2 is joined to an outer pheriphery of an approximately rectangular face plate 4. A phosphor screen 6 is formed on an inner surface of face plate 4. Phosphor screen 6 has regularly arranged red, blue and green phosphor stripes. Each phosphor stripe extends along a direction defined by the short sides of face plate 4. A neck 8 is joined to an end of funnel 2. An electron gun 10 is disposed within neck 8. A deflection apparatus 12 is mounted on an outer surface of funnel 2 and around neck 8. A color selection electrode 14 is mounted within the tube so as to oppose to phosphor screen 6. Color selection electrode 14 includes an aperture portion 16 having a plurality of apertures and is held in place by a frame 18 and resilient support members 20 attached to frame 18. Color selection electrode 14 is attached to pins 22 which are welded to face plate 4. An inner magnetic shield 24 is mounted to color selection electrode 14 and is arranged along an inner wall of funnel 2. Inner magnetic shield 24 is provided for shielding electron beams, generated by electron gun 10, from the earth's magnetic field.
Referring now to FIG. 2, there is shown in perspective view a conventional inner magnetic shield 24. This funnel-shaped inner magnetic is well known as a basic type of inner magnetic shield. Generally inner magnetic shield 24 is made from a ferromagnetic metal plate including iron as its chief component. The thickness of the metal plate is selected to be in the range of 0.1 mm to 0.3 mm for ease in fabrication. However, an inner magnetic shield having such a thickness is too thin to completely shield the electron beams from the magnetic flux. Thus the beam paths are distorted and they strike the wrong points on the inner surface of the tube's face thereby affecting color purity.
One approach to solving this problem was to try to align the direction of lines of magnetic force with the electron beam path or to convert the direction of lines of magnetic force to another direction which does not affect the color purity. The effect of the magnetic shield will be further explained below.
First, the magnetic field component affecting the color purity will be discussed. Generally, a phosphor screen of a color cathode ray tube has a plurality of phosphor stripes continuously extending in the vertical direction of the tube's face plate. Generally, the vertical direction of the face plate corresponds to the direction of the short side of the face plate. Therefore a beam deviation in the vertical direction does not degrade the color purity. For the purposes of this discussion, we will denote the horizontal axis line of the face plate "x", the vertical axis line "y" and the tube axis "z". The magnetic components affecting the color purity are a y axial magnetic field component "By" and a tube axial magnetic field component "Bz". A charged particle in the presence of a magnetic field develops a "Lorentz's force" in accordance with the following relation: EQU F=qv.times.B
in which q is the quantity of electric charge of the particle, v is the velocity of the charged particle and B is magnetic flux density through which the charged particle is traveling. When the charged particle is an electron, the relation is further developed to EQU F=-ev.times.B
in which e is the charge of an electron. The beam deviation in the horizontal direction affects the color purity. Force Fx affecting the electron beam in the horizontal direction is indicated by the following relation: EQU Fx=-e (vy Bz-vz By)
in which vy is a horizontal component of the velocity and vz is a tube axial component of velocity. This relation implies that By and Bz cooperate with vz and vy, respectively to affect the color purity.
Referring now to FIGS. 3A and 3B, the deviation direction of the electron beam is illustrated. Arrows 26 indicate the deviation directions for electron beams in both the upper and lower portions of a color cathode ray tube viewed from the observer side. Both FIGS. 3A and 3B illustrate the cases wherein the color cathode ray tube is located in the northern part of the earth and its face plate is directed north. FIG. 3A shows the electron beam deviation caused by the horizontal component Bz of the earth's magnetic field and the vertical component vy of the electron beam velocity deflected by the deflection apparatus. In the upper part of the screen, the beam is offset to the left. In the lower part of the screen the beam is offset to the right. FIG. 3B shows the electron beam deviation caused by the vertical component By of the earth's magnetic field and the tube's axial component vz of the electron beam velocity. The electron beam is offset to left over the entire screen. These are the effects of beam deviation caused by the earth's magnetic field.
Referring now to FIG. 4, there is shown a known inner magnetic shield 30. Such a shield 30 is disclosed in Japanese Patent Disclosure No. 15001/1978. Magnetic shield 30 has a notch 31 in its short side walls 32. Some magnetic flux components in the z direction, which would be absorbed in the short side wall of prior art magnetic shields (shown in FIG. 2) are directed to the long side walls 33. Accordingly, some horizontal components Bz are converted into vertical components By. That is, vertical component By is increased in the upper part of the screen and vertical component By is decreased in the lower part of the screen. These converted magnetic flux component affect the electron beam deviation in the opposite direction to the deviation caused by the horizontal component Bz. The beam deviation is thereby reduced and the color purity is remarkably increased when the color cathode ray tube is oriented in the north or south direction.
However, when the color cathode ray tube is oriented east or west, the magnetic flux easily passes the region through which the electron beam is passing because of the notches in the side walls. The magnetic flux distribution in that region increases and is deformed by the long side walls 33 of the magnetic shield. The shape of the magnetic flux distribution becomes a barrel shape as shown in FIG. 5. Consequently, the vertical components By are generated at the four corners of the screen as shown in FIG. 5. As a result, a beam deviation toward the center after 35, as shown by arrows 35 occurs in the upper part of the screen, and a beam deviation toward the outside as shown by arrows 36 occurs in the lower part of the screen. A trapezoidal shaped beam miss-landing pattern thereby occurs.
To overcome this drawback, another magnetic shield as shown in FIG. 6 was proposed in Japanese Patent Disclosure No. 13,253/1979. This magnetic shield 38 includes a notch 40 along vertical axis y in the long side wall 41. Notch 40 forms high magnetic resistant portion, which impedes the concentration of magnetic flux in the long side walls 41. Thus the undesirable deformation of the magnetic flux distribution, such as barrel shaped distribution, is prevented. FIG. 7 shows the magnetic field distribution in the inner magnetic shield 38 shown in FIG. 6. This inner magnetic shield has high magnetic resistant parts on vertical axis y. Therefore, the reformation of the local flux distribution is as shown in FIG. 7, and the distribution of the magnetic field acquires harmonic components. On the four corners of the screen the high magnetic resistant part does not have enough effect, so that the same beam deviation as shown in FIG. 5 still remains. However, near the high magnetic resistant part, that is near the vertical axis y the beam deviation direction is opposite to the direction as shown in FIG. 5. This magnetic shield configuration causes local beam deviation near notch 40 and this local beam deviation makes it difficult to cancel the total beam deviation (which includes the beam deviation resulting from other factors) by adjusting the deflection apparatus.
Further, the high magnetic resistant part is required to have enough width to affect the electron beam within the effective screen area, so that additional drawbacks result. When the color cathode ray tube is directed to the north or south, the beam deviation near the high magnetic resistance part becomes large because the weak function of converting the tube axial magnetic field into the vertical magnetic field near the high magnetic resistance part. The resulting beam deviation is also local.
FIG. 8 shows the yet another prior art inner magnetic shield 42 described in Japanese Utility Model Publication No. 27,957/1980. The side wall 44 has openings 45 for forming anisotropy in the magnetic resistance to reduce the demagnetizing power. Opening 45 also contributes to the heat-dispersion of the shadow mask. This inner magnetic shield 42 is not intended to prevent the beam deviation.
As discussed above, prior art inner magnetic shield structures can not render the beam deviation sufficiently small over the entire screen.